1. Field of the Invention
The present invention relates to a displacement generating apparatus using a bimorph-type electrostriction actuator, and more particularly, to an improvement of a method for driving an electrostriction actuator.
2. Description of the Related Art
With the progress of mechatronics, fine displacement control has recently started to be regarded as important. Laminate actuators formed of piezoelectric elements are used in applications which require a very fine displacement. A bimorph-type piezoelectric actuator, which is formed of two piezoelectric boards pasted together, is used for an application which does not require such control of a fine displacement as the laminate actuator needs, but requires a displacement of several hundreds of micrometers or thereabout.
In the bimorph-type piezoelectric actuator constructed in this manner, the hysteretic characteristic of the piezoelectric material itself is reflected in the movement of the distal end portion of the actuator caused by bending, and the relationship between voltage and movement has hysteresis. Thus, although position control utilizing the maximum movement position can be effected relatively accurately, analog position control utilizing an intermediate position of the piezoelectric body on the way to its maximum movement is a very hard task.
Disclosed in Published Examined Japanese Patent Application No. 54-29359 or the like is a method in which hysteresis is suppressed by means of a sensor. In this case, however, the apparatus inevitably has a complicated construction.
On the other hand, electrostriction material has no hysteresis, and its voltage-displacement characteristic is linear. Since the electrostriction material is displaced in proportion to the square of the voltage, however, it never contracts even if a voltage of the opposite direction is applied thereto. In contrast with the conventional method, therefore, a bimorph system is proposed in which displacement control is effected by changing a predetermined voltage previously applied to electrodes on either side of an electrostriction board. There is a demand for the development of a circuit for efficiently driving a bimorph element.